The present invention relates to a linear transducer arrangement for control of a tool carried by a machine, and more specifically, to a control system of a hydraulically moveable tool carried by a machine having laser receivers receiving actual elevational positions of the ends of the tool from an external laser transmitter and a pair of linear transducer arrangement providing relative elevational positions of the ends of the tool to each other.
In concrete paving operations, after concrete is poured it is commonly finished by drawing a tool, such as a screed head, over the surface of the contour to finish the surface of the concrete before it cures. In asphalt paving operations, after asphalt is laid it is commonly leveled to a desired depth by drawing a tool, such as also a screed head of a paver, over the surface of the contour. Finally, in grading operations, a surface is graded to a desired depth by drawing a tool, such as a blade of a grader, over the surface of the contour. Thus, although the physical configurations of the types of screed heads and the grader""s blade are not identical, the functions of these tools are analogous.
Typically, a hydraulic cylinder connected to each end of the tool of the machine, raise and lower the ends of the tool independently. It has been common to determine the elevational positions of the ends of the tool by using a laser transmitter or a sonic pulse as a reference in order to achieve the chosen surface level. As such, the raising and lowering of the tool is controlled by the control system and is in response to reception of the reference signal.
In the laser transmitter arrangement, a projected rotating beam of laser light defines a reference plane. A pair of laser receivers, one receiver mounted at each end of the tool on an associated mast for vertical movement with the tool, detect the reference plane and a control system of the machine then actuates hydraulic valves to supply fluid to the hydraulic cylinders in response to this detected level. As a result, the elevation of each end of the tool can be precisely controlled. In the sonic pulse arrangement, as disclosed by U.S. Pat. No. 4,924,374 to Middleton, et al., a tool carried by a machine, can level a surface to a chosen depth by determining the time it takes for an acoustic pulse to travel from a transducer, such as an ultrasonic receiver, provided on a mast at each end of the tool to a reference surface and back. As a result, with this time value being used to calibrate a microprocessor-controlled distance-measuring device the elevation of each end of the tool can be precisely controlled. Accordingly, in both types of the above described arrangements, each of their respective type of receivers, either laser or sonic, provides elevational feedback to drive the hydraulics controlling the elevation of each side of the tool.
A problem may arise, however, if one the receivers is blocked by something of an appreciable height, such as, for example, a support column in a building, in the case of the laser receiver or interrupted in the case of the ultrasonic receiver. When a blockage or disruption occurs, there is a need to maintain the relative elevation of the ends of the tool until either the laser beam or sonic pulse can be reacquired by both receivers mounted at the ends of the tool. There is also a need to be able to pull the tool along a straight path, while maintaining the chosen thickness of the layer and matching forms or existing surfaces during a screeding, paving, or grading application.
One approach to this problem, in the laser arrangement is to set up two external laser transmitters at the same elevation on opposite sides of the tool. In this way, if a column blocks one of the transmitters, the other external transmitter is likely to be illuminating the receivers at the ends of the tool, thereby compensating for the blockage. Essentially, the prior art method is to eliminate all blind spots around the receivers. However, this prior art method adds an additional cost of a second external transmitter and time to properly set up the second external laser transmitter to eliminate the possibility of a column block.
Another approach to this problem is to use a gravity-based cross slope sensor, which detects the angular shifts of the tool as the tool tilts up and down. Additionally, the gravity-based cross slope sensor may be used as a reference for set up and control in a super flat, or plumb, floor application. Accordingly, when both sides of the tool are within the appropriate dead band, the desired grade of the cross slope sensor is measured and stored in memory of the tool""s control system. When one laser receiver loses reception of the elevational reference, the cross slope sensor detects the height of the interrupted receiver side of the tool relative to its uninterrupted receiver side. That is, the cross slope sensor provides a relative measurement of the interrupted laser receiver which, when coupled with the absolute measurement of the uninterrupted laser receiver, provides an estimate of the absolute position of the interrupted laser receive. The control system of the tool can be used the provided absolute and estimated absolute positions to control the elevation of ends of the tool. However, several disadvantages exist in the cross slope sensor arrangement described above.
First, the gravity-based cross slope sensor is vulnerable to accelerations along its sensitive axis, resulting in miscalculations of the tool""s slope. In screeding operations, it is quite common for the operator to xe2x80x9cside shiftxe2x80x9d the tool around columns as he pulls the tool back. Since the sensitive axis of the gravity-based cross slope sensor is parallel to the length of the tool, this side shifting can cause noticeable acceleration along the sensitive axis of measurement, thus dramatically affecting the feedback of the cross slope sensor. Second, harmonics of the machine boom carrying the tool, which do not cause significant enough elevation shifts to be seen in the laser receivers at both ends of the tool, are detectable by gravity-base cross slope sensor since measuring acceleration and not machine movement. Third, in order to reduce the effects of noise and to compensate for some of the low frequency harmonics of the machine vibration, considerable low pass filtering of the cross slope sensor is required. The use of low pass filters on the output of the gravity-based cross slope sensor adds an inherent time lag to the system, which degrades the bandwidth performance of the blocked side. Finally, separate control gains for the cross slope sensor are used to compensate for the time lag, as well as, the change in loop gain, thus requiring frequent calibration adjustments in order to maintain performance of the control system.
Therefore, there is a need for providing a control system of a hydraulically moveable tool carried by a machine that does not require setting up a second external laser transmitter in order to maintain the elevation of the ends of the tool in a blocked or interrupted receiver situation.
There is also a need for providing a control system of a hydraulically moveable tool carried by a machine that does not require a gravity-based cross slope sensors to maintain the relative elevation of the ends of the tool in a blocked or interrupted situation until the reference signal(s) can be reacquired by both receivers.
These needs are met by a linear transducer arrangement according to the present invention that provides a control signal for use by a conventional control circuit or system of a machine to maintain a selected elevational position between ends of a hydraulically moveable tool carried by the machine and a reference, when reception of the reference by one of a pair of elevation receivers at the ends of the tool is blocked or interrupted until the reference can be reacquired by both elevation receivers. Normally, absolute measurements are available on both side ends of the tool via a pair of mounted elevation receivers, such as laser or ultrasonic receivers. When reception of a reference, such as a laser beam from a laser transmitter or a sonic pulse from a transponder, by one of the of elevation receivers is interrupted, the control signal generated by the linear transducer arrangement of the present invention is used by the machine""s control system to maintain the relative elevation of the side ends of the tool to each other until the reference can be reacquired by both elevation receivers. The present invention assist the control system in controlling the tool in a blocked or interrupted condition since that at any given time, at least one absolute measurement is available for an unblocked or uninterrupted side end of the tool and one relative elevational measurement from that unblocked or uninterrupted side end to the blocked or interrupted side of the tool is available to the control system of the machine. Accordingly, with the generated control signals from the transducer arrangement of the present invention the control system can maintain a relative elevation position of the interrupted receiver side to the absolute position of the uninterrupted receiver side until both receiver can reacquire the elevational reference.
In one aspect, the present invention is a linear transducer arrangement for generating control signals for use by a conventional control circuit or system of a machine, having elevation receivers, in controlling movement of individual hydraulically moveable ends of a tool carried by a machine so as to maintain a selected elevational position between each end of the tool and a reference when reception one of the elevation receivers of the reference is interrupted, the laser transmitter comprising a first linear transducer mounted on a first end of the tool; and a second linear transducer mounted on a second end of the tool, the first and second linear transducers provide electrical outputs indicating the extension of elevation cylinders of the hydraulically moveable ends of the tool, thus providing to the control circuit the relative height of the interrupted elevation receiver to the uninterrupted elevation receiver until the disruption clears.
In another aspect, the present invention is a method of controlling the elevational position of hydraulically moveable ends of a tool of a machine in relationship to a reference detected by elevation receivers attached the ends of the tool, when reception of one of the elevation receivers of the reference is interrupted, comprising the steps of selecting a desired elevational position of the tool to the reference with the elevation receivers; generating outputs with a pair of linear transducers, each of the pair of linear transducers is associated with an elevation cylinder at each of the hydraulically moveable ends of the tool, and each of the outputs indicating the extension of the associated elevation cylinder; and using the output of the linear transducer associated with the hydraulically moveable end having the interrupted elevation receiver to maintain a constant relative height between the hydraulically moveable ends until the disruption clears.
Other objects, features and advantages will appear more fully in the course of the following discussion.